Durable Nano-Flower Structured Foam Coupled with Electrically-Driven in Situ Aeration Enable High-Flux Oil/Water Emulsion Separation with Dynamic Antifouling Ability.

The conventional membranes used for separating oil/water emulsions are typically limited by the properties of the membrane materials and the impact of membrane fouling, making continuous long-term usage unachievable. In this study, a filtering electrode with synchronous self-cleaning functionality is devised, exhibiting notable antifouling ability and an extended operational lifespan, suitable for the continuous separation of oil/water emulsions. Compared with the original Ti foam, the in situ growth of NiTi-LDH (Layered double hydroxide) nano-flowers endows the modified Ti foam (NiTi-LDH/TF) with exceptional superhydrophilicity and underwater superoleophobicity. Driven by gravity, a rejection rate of over 99% is achieved for various emulsions containing oil content ranging from 1% to 50%, as well as oil/seawater emulsions. The flux recovery rate exceeds 90% after one hundred cycles and a 4-h filtration period. The enhanced separation performance is realized through the "gas bridge" effect during in situ aeration and electrochemical anodic oxidation. The internal aeration within the membrane pores contributes to the removal of oil foulants. This study underscores the potential of coupling foam metal filtration materials with electrochemical technology, providing a paradigm for the exploration of novel oil/water separation membranes.

Thiol-Ene Click Reaction in Constructing Liquid Separation Membranes for Water Treatment.

In the evolving landscape of water treatment, membrane technology has ascended to an instrumental role, underscored by its unmatched efficacy and ubiquity. Diverse synthesis and modification techniques are employed to fabricate state-of-the-art liquid separation membranes. Click reactions, distinguished by their rapid kinetics, minimal byproduct generation, and simple reaction condition, emerge as a potent paradigm for devising eco-functional materials. While the metal-free thiol-ene click reaction is acknowledged as a viable approach for membrane material innovation, a systematic elucidation of its applicability in liquid separation membrane development remains conspicuously absent. This review elucidates the pre-functionalization strategies of substrate materials tailored for thiol-ene reactions, notably highlighting thiolation and introducing unsaturated moieties. The consequential implications of thiol-ene reactions on membrane properties-including trade-off effect, surface wettability, and antifouling property-are discussed. The application of thiol-ene reaction in fabricating various liquid separation membranes for different water treatment processes, including wastewater treatment, oil/water separation, and ion separation, are reviewed. Finally, the prospects of thiol-ene reaction in designing novel liquid separation membrane, including pre-functionalization, products prediction, and solute-solute separation membrane, are proposed. This review endeavors to furnish invaluable insights, paving the way for expanding the horizons of thiol-ene reaction application in liquid separation membrane fabrication.

A synergistic approach combining computational fluid dynamics simulation with hydrolysis-acidification for dye wastewater treatment.

Wastewater treatment is effectively conducted using anaerobic biological methods. Nevertheless, the efficiency of these methods can be hindered by challenges like short-circuits and dead zones, particularly in treating persistent contaminants. This work utilized computational fluid dynamics (CFD) simulations to enhance water distribution, ensuring uniform interactions between solid and liquid phases, and thus mitigating issues related to short-circuits and dead zones. Such enhancements notably amplified the anaerobic biological process's efficiency. Furthermore, dye biodegradability was improved through the application of the hydrolysis acidification technique. Optimal hydraulic retention time for the hydrolysis-acidification reactor, established at 9 h, was determined via sludge cultivation and domestication for stable operation. During stable operation, an elevation in effluent volatile fatty acids was observed, alongside a COD removal rate fluctuating between 15% and 29%. Approximately 50% was noted as the rate of color removal. Simultaneously, a noticeable decrease in effluent pH occurred, with total nitrogen removal approximating 8%. An estimated BOD5/COD ratio of 0.32 was recorded. The incorporation of microbial agents led to an enhanced COD removal, ranging from 28% to 33%, thereby stabilizing the effluent BOD5/COD ratio at around 0.35. This research highlights the advantages of optimizing water distribution in anaerobic reactors, particularly when combined with hydrolysis-acidification techniques, effectively addressing issues of short-circuits and dead zones.

Enhanced sludge dewatering using a novel synergistic iron/peroxymonosulfate-polyacrylamide method.

In the sludge dewatering process, a formidable challenge arises due to the robust interactions between extracellular polymeric substances (EPS) and bound water. This study introduces a novel, synergistic conditioning method that combines iron (Fe2+)/peroxymonosulfate (PMS) and polyacrylamide (PAM) to significantly enhance sludge dewatering efficiency. The application of the Fe2+/PMS-PAM conditioning method led to a substantial reduction in specific filtration resistance (SFR) by 82.75% and capillary suction time (CST) by 80.44%, marking a considerable improvement in dewatering performance. Comprehensive analyses revealed that pre-oxidation with Fe2+/PMS in the Fe2+/PMS-PAM process effectively degraded EPS, facilitating the release of bound water. Subsequently, PAM enhanced the flocculation of fine sludge particles resulting from the advanced oxidation processes (AOPs). Furthermore, analysis based on the Extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory demonstrated shifts in interaction energies, highlighting the breakdown of energy barriers within the sludge and a transition in surface characteristics from hydrophilic (3.79 mJ m-2) to hydrophobic (-61.86 mJ m-2). This shift promoted the spontaneous aggregation of sludge particles. The innovative use of the Flory-Huggins theory provided insights into the sludge filtration mechanism from a chemical potential perspective, linking these changes to SFR. The introduction of Fe2+/PMS-PAM conditioning disrupted the uniformity of the EPS-formed gel layer, significantly reducing the chemical potential difference between the permeate and the water in the gel layer, leading to a lower SFR and enhanced dewatering performance. This thermodynamic approach significantly enhances our understanding of sludge dewatering and conditioning. These findings represent a paradigm shift, offering innovative strategies for sludge treatment and expanding our comprehension of dewatering and conditioning techniques.

Novel thermodynamic mechanisms of co-conditioning with polymeric aluminum chloride and polyacrylamide for improved sludge dewatering: A paradigm shift in the field.

This study investigated the combined effects of polymeric aluminum chloride (PAC) and polyacrylamide (PAM) on sludge dewatering, aiming to unveil underlying mechanisms. Co-conditioning with 15 mg g-1 PAC and 1 mg g-1 PAM achieved optimal dewatering, reducing specific filtration resistance (SFR) of co-conditioned sludge to 4.38 × 1012 m-1kg-1, a mere 48.1% of raw sludge's SFR. Compared with the CST of raw sludge (36.45 s), sludge sample can be significantly reduced to 17.7 s. Characterization tests showed enhanced neutralization and agglomeration in co-conditioned sludge. Theoretical calculations revealed elimination of interaction energy barriers between sludge particles post co-conditioning, converting sludge surface from hydrophilic (3.03 mJ m-2) to hydrophobic (-46.20 mJ m-2), facilitating spontaneous agglomeration. Findings explain improved dewatering performance. Based on Flory-Huggins lattice theory, connection between polymer structure and SFR was established. Raw sludge formation triggered significant change in chemical potential, increasing bound water retention capacity and SFR. In contrast, co-conditioned sludge exhibited thinnest gel layer, reducing SFR and significantly improving dewatering. These findings represent a paradigm shift, shedding new light on fundamental thermodynamic mechanisms of sludge dewatering with different chemical conditioning.

Predicting thermodynamic adhesion energies of membrane fouling in planktonic anammox MBR via backpropagation neural network model.

Predicting thermodynamic adhesion energies was a critical strategy for mitigating membrane fouling. This study utilized a backpropagation (BP) neural network model to predict the thermodynamic adhesion energies associated with membrane fouling in a planktonic anammox MBR. Acid-base (ΔGAB), electrostatic double layer (ΔGEL), and Lifshitz-van der Waals (ΔGLW) energies were selected as output variables, the training dataset was collected by the advanced Derjaguin-Landau-Verwey-Overbeek (XDLVO) method. Optimization results identified "7-10-3″ as the optimal network structure for the BP model. The prediction results demonstrated a high degree of fit between the predicted and experimental values of thermodynamic adhesion energy (R2 ≥ 0.9278), indicating a robust predictive capability of the model in this study. Overall, the study presented a practical BP neural network model for predicting thermodynamic adhesion energies, significantly enhancing the prediction tool for adhesive fouling behavior in anammox MBRs.

Innovative treatment of industrial effluents through combining ferric iron and attapulgite application.

The escalation of industrial activities has escalated the production of pharmaceutical and dyeing effluents, raising significant environmental issues. In this investigation, a hybrid approach of Fenton-like reactions and adsorption was used for deep treatment of these effluents, focusing on effects of variables like hydrogen peroxide concentration, catalyst type, pH, reaction duration, temperature, and adsorbent quantity on treatment effectiveness, and the efficacy of acid-modified attapulgite (AMATP) and ferric iron (Fe(III))-loaded AMATP (Fe(III)-AMATP) was examined. Optimal operational conditions were determined, and the possibility of reusing the catalysts was explored. Employing Fe3O4 as a heterogeneous catalyst and AMATP for adsorption, CODCr was reduced by 78.38-79.14%, total nitrogen by 71.53-77.43%, and phosphorus by 97.74-98.10% in pharmaceutical effluents. Similarly, for dyeing effluents, Fe(III)-AMATP achieved 79.87-80.94% CODCr, 68.59-70.93% total nitrogen, and 79.31-83.33% phosphorus reduction. Regeneration experiments revealed that Fe3O4 maintained 59.48% efficiency over three cycles, and Fe(III)-AMATP maintained 62.47% efficiency over four cycles. This work offers an economical, hybrid approach for effective pharmaceutical and dyeing effluent treatment, with broad application potential.

Harnessing diurnal dynamics: Deciphering the interplay of light cycles on algal-bacterial membrane bioreactors.

Light profoundly modulates the algal-bacterial membrane bioreactor (algal-bacterial MBR) performance. Yet, its outdoor deployment grapples with the inherent diurnal cycle of sunlight, engendering suboptimal light conditions. The adaptability of such systems to these fluctuating light conditions and their implications for practical outdoor applications remained an under-explored frontier. In response, this study meticulously scrutinized two laboratory-scale algal-bacterial MBRs under varying light regimes: a 24-h continuous and a 12-h cyclic illumination. Over 70 days, continuous illumination was observed to yield superior biomass production and total nitrogen and total phosphorus removal efficiencies compared to its cyclic counterpart. Contrarily, when focusing on membrane fouling, the 12-h cyclic illumination exhibited lower membrane fouling. The spectral analyses coupled with adhesion ability evaluation, traced the enhanced membrane fouling under continuous illumination to the elevated organics and heightened adhesive properties of the flocs. Given the tangible benefits of reduced membrane fouling and the potential harnessing of solar radiation, the 12-h cyclic illumination emerges as an economically astute operational paradigm for algal-bacterial MBRs. The significance of this study is to promote the application of algal-bacterial MBR in sewage treatment and provide robust support for the development of green technology in the future.

Mg2+ addition: Unlocking optimized treatment performance and anti-fouling property in microalgal-bacterial membrane bioreactors.

While microalgal-bacterial membrane bioreactors (microalgal-bacterial MBRs) have risen as an important technique in the realm of sustainable wastewater treatment, the membrane fouling caused by free microalgae is still a significant challenge to cost-effective operation of the microalgal-bacterial MBRs. Addressing this imperative, the current study investigated the influence of magnesium ion (Mg2+) addition on the biological dynamics and membrane fouling characteristics of the laboratory-scale submerged microalgal-bacterial MBRs. The results showed that Mg2+, important in augmenting photosynthetic process, yielded a biomass concentration of 2.92 ± 0.06 g/L and chlorophyll-a/MLSS (mixed liquor suspended solids) of 33.95 ± 1.44 mg/g in the RMg (Mg2+ addition test group). Such augmentation culminated in elevated total nitrogen and phosphorus removal efficiencies, clocking 81.73 % and 80.98 % respectively in RMg. Remarkably, despite the enhanced microalgae activity and concentration in RMg, the TMP growth rate declined by a significant 46.8 % compared to R0. Detailed characterizations attributed reduced membrane fouling of RMg to a synergy of enlarged floc size and reduced EPS contents. This transformation is intrinsically linked to the bridging action of Mg2+ and its role in creating a non-stressed ecological environment for the microalgal-bacterial MBR. In conclusion, the addition of Mg2+ in the microalgal-bacterial MBR appears an efficient approach, improving the efficiency of pollutant treatment and mitigating fouling, which potentially revolutionizes cost-effective applications and propels the broader acceptance of microalgal-bacterial MBRs. It also of great importance to promote the development and application of microalgal-bacterial wastewater treatment technology.

Mxenes for membrane separation: from fabrication strategies to advanced applications.

Transition metal carbides/nitrides/carbonitrides, commonly referred to as MXenes, have gained widespread attention since their discovery in 2011 as a promising family of two-dimensional (2D) materials. Their impressive chemical, electrical, thermal, mechanical, and biological properties have fueled a surge in research focused on the synthesis and application of MXenes in various fields, including membrane-based separation. By engineering the materials and membrane structures, MXene-based membranes have demonstrated remarkable separation performance and added functionalities, such as antifouling and photocatalytic properties. In this review, we aim to have a timely and critical review of research on their fabrication strategy and performance in advanced molecular separation and ion exchange, beginning with a brief introduction of the preparation and physicochemical properties of MXenes. Finally, outlooks and future works are outlined with the aims to provide valuable insights and guidance for advancing membranes' applications in different separation domains.

An efficient solution based on the synergistic effects of nickel foam in NiFe-LDH nanosheets for oil/water separation.

Efficient oil-water separation has always been a research hotspot in the field of environmental studies. Employing a one-step hydrothermal approach, NiFe-layered double hydroxides (LDH) nanosheets were synthesized on nickel foam substrates. The resulting NiFe-LDH/NF membrane exhibited rejection rates exceeding 99% across six diverse oil-water mixtures, concurrently demonstrating a remarkable ultra-high flux of 1.4 × 106 L·m-2·h-1. This flux value significantly surpasses those documented in existing literature, maintaining stable performance over 1000 manual filtration cycles. These breakthroughs stem from the synergistic interplay among the three-dimensional channels of the nickel foam, the nanosheets, and the hydration layer. By leveraging the pore size of the foam to enhance the functionality of the hydration layer, the conventional trade-off between permeability and selectivity was transformed into a balanced force relationship between the hydration layer and the oil phase. The operational and failure mechanisms of the hydration layer were examined using the prepared NiFe-LDH/NF membrane, validating the correlation between oil phase viscosity and density with hydration layer rupture. Additionally, an extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory was employed to investigate changes in interaction energy, further reinforcing the study's findings. This research contributes novel insights and assistance to the comprehension and application of hydration layers in other membrane studies dedicated to oil-water separation.

Mitigation of protein fouling by magnesium ions and the related mechanisms in ultrafiltration process.

Although protein is an important membrane foulant in the water body that may be significantly affected by the coexisting common cation magnesium (Mg2+), the effect of Mg2+ on protein fouling is rarely reported. In this context, this study selected bovine serum albumin (BSA) as the model foulant, and investigated its fouling characteristics at different Mg2+ concentrations (0-100 mM). Filtration tests showed that the protein fouling can be significantly mitigated by adding Mg2+, and the specific filtration resistance (SFR) of pure BSA (3.56 × 1014 m kg-1) was at least 5 times that of BSA-Mg2+ solutions (0.5-100 mM). In addition, an optimal Mg2+ concentration exists, which can achieve the lowest BSA SFR. A series of characterizations indicated that the main contributors to the differences in BSA SFR were the changes in BSA adhesion capacity and the thickness and structure of the foulant layer. Basically, the above results were attributed to the hydration repulsion effect of Mg2+, which prevented tight adhesion of foulants to the membrane. Moreover, the lowest BSR SFR at 1 mM Mg2+ was achieved not only by the hydration repulsion effect but also by the particle size compression due to the conformational change of BSA molecules. This combined effect led to the lowest foulant retention on the membrane surface and delivered to the lowest SFR. This study conducts a thorough inspection into the specific effect of Mg2+ on protein fouling and provides a fresh insight into protein fouling control in the UF process.

Optimizing aeration intensity to enhance self-flocculation in algal-bacterial symbiosis systems.

Effectuating optimal wastewater treatment via algae-bacterial symbiosis (ABS) systems necessitates the precise selection of aeration intensity. This study pioneers an in-depth investigation into the interplay of aeration intensity on the microalgal-bacterial consortia's self-flocculation efficacy and the overall treatment performance within ABS systems. The research provides evidence for a direct association between aeration intensity and biomass proliferation, indicating enhanced pollutant removal efficiency with escalated intensities (1.0 and 1.5 L min-1), though the variance lacks statistical significance. The peak self-flocculation efficacy of the microalgal-bacterial consortium (82.39% at 30 min) was manifested at an aeration intensity of 1.0 L min-1. The meticulous analysis of biomass properties showed the complexity of self-flocculation capacity in the consortium, which involves a dynamic interplay of several pivotal factors, including floc size, zeta potential, and EPS content. In situations where these factors pose conflicting influences, the determining factor emerges as the dominant influencer. In this study, the optimal aeration intensity was identified as 1 L min-1, shedding light on the critical threshold for ABS system operation. This study not only enriches the understanding of microalgal-bacterial wastewater treatment mechanisms but also fosters innovative strategies to enhance the performance of such systems.

3D printing titanium dioxide-acrylonitrile-butadiene-styrene (TiO2-ABS) composite membrane for efficient oil/water separation.

The oily water treatment is becoming one of the hottest topics due to that increase of offshore oil transportation and the various accident oil leakages. In this study, a functional TiO2-ABS composite membrane was generated through the three-dimensional (3D) printing strategy for the first time and was conducted to simulated oily water treatment. The TiO2-ABS composite membrane demonstrated a significant promotion in hydrophilicity and oleophobicity which were evidenced by the water contact angle of 14.8° and the underwater oil contact angle of 144.7°, respectively. The optimal modified membrane had both exceedingly high flux (1.8 × 105 L m-2·h-1) and oil rejection rate (99.5%). Moreover, the results of filtration cycles of 10 days and extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory demonstrated that the modified membranes took possession of excellent stability and antifouling property. What was more, the TiO2-ABS composite membrane revealed over 99% rejection to all five types of oil/water systems. The interestingly experimental results indicated that the prepared membrane possessed a broad development trend and application prospect in the field of oily water treatment.

Superior performance of a membrane bioreactor through innovative in-situ aeration and structural optimization using computational fluid dynamics.

The optimization of membrane bioreactors (MBRs) involves a critical challenge in structural design for mitigation of membrane fouling. To address this issue, a three-dimensional computational fluid dynamics (CFD) model was utilized in this study to simulate the hydrodynamic characteristics of a flat sheet (FS) MBR. The optimization of the membrane module configuration and operating conditions was performed by investigating key parameters that altered the shear stress and liquid velocity. The mixed liquor suspended solids (MLSS) concentration was found to increase the shear stress, leading to a more uniform distribution of shear stress. By optimizing the appropriate bubble diameter to 5 mm, the shear stress on the membrane surface was optimized with relatively uniform distribution. Additionally, extending the side baffle length dramatically improved the uniformity of the shear stress distribution on each membrane. A novel in-situ aeration method was also discovered to promote turbulent kinetic energy by 200 times compared with traditional aeration modes, leading to a more uniform bubble streamline. As a result, the novel in-situ aeration method demonstrated superior membrane antifouling potential in the MBR. This work provides a new approach for the structural design and optimization of MBRs. The innovative combination of the CFD model, optimization techniques, and novel in-situ aeration method has provided a substantial contribution to the advancement of membrane separation technology in wastewater treatment.

Quantifying interfacial interactions for improved membrane antifouling: A novel approach using triangulation and surface element integration method.

To gain a thorough understanding of interfacial behaviors such as adhesion and flocculation controlling membrane fouling, it is necessary to simulate the actual membrane surface morphology and quantify interfacial interactions. In this work, a new method integrating the rough membrane morphology reconstruction technology (atomic force microscopy (AFM) combining with triangulation technique), the surface element integration (SEI) method, the extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory, the compound Simpson's approach, and the computer programming was proposed. This new method can exactly mimic the real membrane surface in terms of roughness and shape, breaking the limitation of previous fractal theory and Gaussian method where the simulated membrane surface is only statistically similar to the real rough surface, thus achieving a precise description of the interfacial interactions between sludge foulants and the real membrane surface. This method was then applied to assess the antifouling propensity of a polyvinylidene fluoride (PVDF) membrane modified with Ni-ZnO particles (NZPs). The simulated results showed that the interfacial interactions between sludge foulants in a membrane bioreactor (MBR) and the modified PVDF-NZPs membrane transformed from an attractive force to a repulsive force. The phenomenon confirmed the significant antifouling propensity of the PVDF-NZPs membrane, which is highly consistent with the experimental findings and the interfacial interactions described in previous literature, suggesting the high feasibility and reliability of the proposed method. Meanwhile, the original programming code of the quantification was also developed, which further facilitates the widespread use of this method and enhances the value of this work.

A unified thermodynamic fouling mechanism based on forward osmosis membrane unique properties: An asymmetric structure and reverse solute diffusion.

Fouling mechanism of the forward osmosis membrane, which was peculiarly featured by the asymmetric membrane structure and reverse solute diffusion, was investigated at the molecular level and from the energy perspective. Two noteworthy fouling behaviors were observed in batch fouling tests conducted in AL-FS mode (active layer facing feed solution) and AL-DS mode (active layer facing draw solution) after filtering foulants with identical volume: 1) after filtering 100 mL of foulants, the flux decline rate in AL-DS mode was 1.78 times faster than that in AL-FS mode, but the flux decline behaviors of the two modes were similar in the subsequent filtration stages; 2) although the foulant layer weight of the same mode increased linearly in middle and late stages, the flux loss rate was distinctly different. Thermodynamic analysis indicated that the attractive interaction energy between the foulants and the support layer was about 5 times higher than that between the foulants and the active layer, well interpreting the higher flux decline rate of AL-DS mode in initial stage. Meanwhile, a non-invasive microscope observed that the structure of the fouling layer remarkably changed from loose to dense in the middle stage, and stabilized in the late stage. Furthermore, quantum chemistry calculation proved that the reverse diffusion of NaCl brought alginate molecular chains closer, whereas the distance between them tended to be constant as the continuous increase of NaCl. Based on these findings, the thermodynamic fouling mechanism proposed by combining the structure change process of the fouling layer with Flory-Huggins lattice theory satisfactorily interpreted the noteworthy fouling behaviors caused by reverse NaCl diffusion in middle and late stages. The revealed fouling mechanism unifies the adhesion and filtration behaviors related to the unique properties of FO membrane, deepening understanding of membrane fouling in the dynamic and complex ternary system of the FO process.

Mutual effects of CO2 absorption and H2-mediated electromethanogenesis triggering efficient biogas upgrading.

Anaerobic digestion coupled with bioelectrochemical system (BES) is a promising approach for biogas upgrading with low energy input. However, the alkalinity generation from electromethanogenesis is invariably ignored which could serve as a potential assistant for CO2 removal through the transformation into dissolved inorganic carbon (DIC). Herein, a novel bioelectrochemical CO2 conversion in the methanogenic BES was proposed based on active CO2 capture and in-situ microbial utilization. It was found that the BES using a stainless steel/carbon felt hybrid biocathode (BES-SSCF reactor) achieved a CH4 yield of 0.33 ± 0.03 LCH4/gCODremoval and increased CH4 production rate by 28.3% of BES-CF reactor at 1.0 V applied voltage. As the experiment progressed, CH4 content increased to 93.1% and CO2 content in the upgraded biogas maintained at below 3%. The continuous proton consumption from H2 evolution reaction in the hybrid biocathode was capable of creating a slightly alkaline condition in the BES-SSCF reactor and thereby the CO2 capture as bicarbonate was enhanced through endogenous alkalinity absorption. Microbial community analysis revealed that significant enrichment of Methanobacterium and Methanosarcina at the BES-SSCF cathodic biofilm was favorable for bicarbonate reduction into CH4 via establishment of H2-mediated electron transfer. Consequently, the remained CO2 and DIC only accounted for 12% of total carbon in the BES-SSCF reactor and the high conversion rate of CO2 to CH4 (82.3%) was achieved. These results unraveled an innovative CO2 utilization mechanism integrating CO2 absorption with H2-mediated electromethanogenesis.

Fundamental thermodynamic mechanisms of membrane fouling caused by transparent exopolymer particles (TEP) in water treatment.

While transparent exopolymer particles (TEP) has high fouling potential, its underlying fouling mechanisms have not yet been well revealed. In current work, fouling characteristics of TEP under different Ca2+ concentrations (0 to 1.5 mM) were investigated. TEP quantification and filtration tests showed that TEP contents increased with Ca2+ concentration, while TEP's specific filtration resistance (SFR) under the influence of Ca2+ concentration presented a unimodal pattern. The peak of TEP's SFR reached at Ca2+ concentration of 1 mM when SA concentration was 0.3 g·L-1. A series of characterizations suggested that microstructure transformation of TEP particles was the main contributor to the resistance variations of TEP solution. The optical microscope observation showed that above and below the critical Ca2+ concentration (1 mM when SA concentration is 0.3 g·L-1 in this study), the formed TEP existed in the form of c-TEP (average particle size is 0.24 µm) and p-TEP (average particle size is 1.05 µm), respectively. Thermodynamic analysis showed that the adhesion ability of c-TEP (-249,989 and - 303,692 kT) was more than 19 times than that of p-TEP (-12,905 kT), which would accelerate foulant layer formation. In addition, below the critical value, the increased SFR with Ca2+ concentration could be explained by integrating Flory-Huggins lattice theory with the preferential intermolecular coordination. Above the critical value, the decreased SFR can be attributed to the formation of a "large-size crack structure" cake layer from the p-TEP. This study revealed fundamental mechanisms of membrane fouling caused by TEP, greatly deepening understanding of TEP fouling, and facilitating to development of effective fouling control strategies.

Molecular-level insights into the mitigation of magnesium-natural organic matter induced ultrafiltration membrane fouling by high-dose calcium based on DFT calculation.

While magnesium cation (Mg2+) universally coexists with natural organic matter (NOM) in the water environment, influence of Mg2+ on NOM fouling in membrane filtration process is still unclear. This work was therefore performed to investigate effects of Mg2+ on NOM (sodium alginate (SA) as a model substance) fouling and role of Ca2+ in mitigating fouling from Mg2+ in the ultrafiltration (UF) water treatment process. Filtration tests showed two interesting fouling phenomena: (1) membrane fouling caused by combination of Mg2+ and SA maintained at a high value with the increased Mg2+ concentration; (2) the high fouling property of Mg2+ can be significantly improved by the prominent addition of calcium cation (Ca2+). It was found that changes of foulant morphology played essential roles through thermodynamic mechanisms represented by the Flory-Huggins lattice theory. Density functional theory (DFT) calculation showed that the combination of SA and Mg2+ tends to coordinate two terminal carboxyl groups in SA, beneficial to stretching alginate chains and forming a stable gel network at low doses. In addition, intramolecular coordination is difficult to occur between SA and Mg2+ due to the high hydration repulsion radius of Mg2+. Therefore, a dense and thick gel network remained even under high Mg2+concentration. Furthermore, due to the higher binding affinity of Ca2+ over Mg2+, high doses of Ca2+ trigger a transition of the stable SA-Mg2+ gel network to other configurations where flocculation and aggregation occur, thereby reducing the specific filtration resistance. The proposed thermodynamic mechanism satisfactorily explained the above interesting fouling behaviors, facilitating to development of new solutions to control membrane fouling.